Polymer-bound sensitizer

ABSTRACT

A photoresist composition for use in lithographic processes in the fabrication of semiconductor devices such as integrated circuit structures is disclosed. The photoresist composition includes a monomeric sensitizer bounding to a base-soluble long chain polymer.

TECHNICAL FIELD

This invention relates generally to a photoresist composition for use inthe manufacture of semiconductor integrated circuits, and morespecifically to a photoresist composition suitable for use as: (i) asensitizer for lithographic processes, including exposure to such energysources as ultraviolet light, X-rays, ion beams and electron beams; ii)a soluble dye for deep ultraviolet positive or negative photoresists;and (iii) a polymer backbone in a terpolymer acid labile photoresistformulation.

BACKGROUND OF THE INVENTION

Lithographic processes are used in the manufacture of semiconductordevices, such as integrated circuit chips. The processes typicallyinvolve the steps of depositing a layer of a photoresist material, e.g.,one which will react when exposed to light, onto a device substrate,such as a circuit chip wafer, by means such as spin-apply; baking thedevice, typically at a temperature of approximately 110° Celsius (C),although high bakes can occur at temperatures above 140° C., to set thephotoresist layer; selectively exposing portions of the photoresistlayer to light, e.g. ultraviolet (UV) light, or other ionizingradiation, e.g. X-rays and ion or electron beams; and developing thephotoresist layer by washing with a basic developer solution, e.g.tetramethylammonium hydroxide (TMAH), thereby removing thenon-irradiated (as in a negative photoresist) or the irradiated (as in apositive photoresist) portions of the photoresist layer. This process,along with typical photoresist materials are described in SemiconductorLithography, W. Moreau, Plenum Press, 1989, which is incorporated hereinby reference.

The photoresist material is formulated by dissolving a polymer resin, aphotoacid generator and a cross-linker in a casting solvent. Aphotosensitizing additive, commonly referred to as a sensitizer, isadded to the formulation to increase the photosensitivity of thephotoresist formulation. By varying the amount of sensitizer added tothe photoresist, the photospeed of the development process can bemodulated.

An important technical limitation of existing sensitizers is that theyare not highly soluble in photoresist casting solvents or developersolutions, and consequently, the concentration of sensitizer that can beemployed in the photoresist formulation is limited. Secondly, existingsensitizers are susceptible to sublimation during the baking process,thereby depleting the photoresist formulation of sensitizer. Inaddition, the sublimed sensitizer can coat the baking tools and thenflake off during the subsequent process, redepositing on the device,resulting in further problems in the system.

While various sensitizers have been developed for use in photoresistformulations, the problems caused by their low solubility in photoresistcasting solvents and developer solutions have not been addressed.

For example, U.S. Pat. No. 4,371,605, issued Feb. 1, 1983 to Renner andassigned to E. I. DuPont de Nemours and Company, discloses severalanthracene derivatives that may be used as a sensitizer in theformulation of a photopolymerizable composition. The patent does not,however, address the problems caused by the sensitizers' low solubility.

U.S. Pat. No. 5,296,332, issued Mar. 22, 1994 to Sachdev et al. andassigned to International Business Machines Corporation, discloseshigh-sensitivity, high contrast, heat-stable photoresist compositionsfor use in deep UV, i-line e-beam and x-ray lithography. While thecompositions disclosed are aqueous base developable, the potentialproblems caused by the sublimation of sensitizer and the deposition ofthe sublimed sensitizer as a physical precipitate on the semiconductordevice remain unresolved.

Therefore, there exists a need to develop a sensitizer that is highlysoluble in photoresist casting solvents and developer solutions,allowing the sensitizer to be used in greater concentration in thephotoresist formulation, without the sublimation of the sensitizerduring the baking process and the subsequent precipitation of thesublimed sensitizer during the development process.

SUMMARY OF THE INVENTION

A composition suitable for use as a sensitizer in a photoresistformulation for lithographic processes is disclosed. The compositioncomprises a typical monomeric sensitizer, e.g. 9-anthracene methanol,and a base-soluble long chain polymer backbone, e.g.poly-4-hydroxystyrene (PHS). The monomeric sensitizer is chemicallybound to the polymer backbone to yield a polymer-bound sensitizer (PBS).As the polymer backbone is highly base-soluble, the resultant PBS ishighly soluble in photoresist casting solvents and developer solutions.In addition, because varying amounts of the monomeric sensitizer can bebound to the polymer backbone, the concentration of sensitizer employedin the photoresist formulation is no longer limited. Furthermore, thePBS provides exposure and sensitization properties equivalent to thoseof the unbound monomeric sensitizer.

Numerous other advantages and features of the present invention willbecome readily apparent from the following detailed description of thepreferred embodiment, the accompanying examples and the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Although this invention is susceptible to embodiment in many differentforms, preferred embodiments of the invention are shown. It should beunderstood, however, that the present disclosure is to be considered asan exemplification of the principles of this invention and is notintended to limit the invention to the embodiments illustrated.

A photoresist composition according to the present invention has severalapplications. First, is as a sensitizer in a photoresist formulation.The composition is highly soluble in both positive and negativephotoresist casting solvents and in metal-ion free developer solutions,such as tetramethylammonium hydroxide (TMAH), thereby eliminatingsublimation of the sensitizer during the baking process and thedeposition of the sublimed sensitizer as a physical precipitate duringthe subsequent process. The composition, which in this application isreferred to as a polymer-bound sensitizer (PBS), comprises a monomericsensitizer bound to a base-soluble long chain polymer having a molecularweight within the range of 1000 to 250,000. The concentration ofmonomeric sensitizer that can be incorporated into the PBS is within therange of 1 to 25 mole percent (mole %), and preferably within the rangeof 1 to 15 mole %. The preferred method for incorporating the monomericsensitizer and the polymer is an acid catalyzed alkylation mechanismsuch as a Friedel-Crafts acylation type reaction wherein the hydroxy(—OH) group remains intact, i.e., it is not replaced by the monomericsensitizer.

Generally, a photoresist formulation includes a polymer resin, aphotoacid generator (PAG), and a cross-linker, dissolved in a castingsolvent. Optionally, a sensitizer is added to increase the formulation'sphotosensitivity. In a photoresist formulation according to oneembodiment of the present invention, a PBS functions as the sensitizer.In an alternate embodiment of the present invention, the PBS canfunction as both sensitizer and the polymer resin. However, as thepurpose of the polymer resin is to enhance the development of thephotoresist material and to ensure development times of less than oneminute, the PBS can only function as both species when realisticdevelopment times can be achieved without the inclusion of the addedpolymer resin.

In forming a semiconductor device such as an integrated circuit (IC)chip, the photoresist formulation is deposited on the device and thenexposed to a chosen energy source, such as UV light, X-rays, or ion orelectron beams. The PAG of the photoresist formulation reacts to theenergy source and generates acid, which in turn causes polymerizationreactions in the polymer resin. The sensitizer aids in this process byreacting with the energy source to initiate the photo-reaction of thePAG. The cross-linker functions to create increased molecular weightpolymers in the photoresist formulation. Portions of the photoresistformulation which are not polymerized are subsequently removed from thephotoresist layer on the device with developer solution.

Examples of monomeric sensitizers that may be incorporated into the PBScomposition include chrysenes, pyrenes, fluoranthenes, anthrones,benzophenones, thioxanthones and anthracenes, such as 9-anthracenemethanol (ANME). Additional anthracene derivative sensitizers aredisclosed in U.S. Pat. No. 4,371,605, which is incorporated herein byreference. The sensitizer may include oxygen or sulfur, however, thepreferred sensitizers will be nitrogen-free, as the presence ofnitrogen, e.g. an amine or phenothiazine group, tends to sequester thefree acid generated during the exposure process and the formulation willlose photosensitivity.

Preferred polymer backbones for the PBS composition include any of thebase-soluble long chain polymers suitable for use as a polymer resin inthe photoresist formulation. Specific examples include: (i) aromaticpolymers having an —OH group, e.g. polyhydroxystyrenes, such aspoly-4-hydroxystyrene (PHS) (available from Maruzen America, New York,N. Y. and Hoechst Celanese of Corpus Christi, Tex.), novolak resins(available from Shipley of Marlboro, Mass.) and polymers having a phenol—OH group, e.g. phenol formaldehyde resins; (ii) polymers having an acidgroup, e.g. polymethacrylic acid with an ester side chain; and (iii)acrylamide group type polymers.

The polymer resin is base-soluble and compatible with developersolutions, such as aqueous solutions of metal-free ammonium hydroxide,tetramethylammonium hydroxide (TMAH), and tetraethylammonium hydroxide;metal containing potassium hydroxide; and sodium metasilicate. Preferredpolymer resins have an average molecular weight within the range of 1000to 250,000, and preferably within the range of 1000 to 25,000, toenhance its solubility in developer solutions. Examples includep-hydroxystyrene-maleic anhydride copolymers,polyhydroxystyrene-p-tertiarybutyl-carbanatostyrene copolymers,poly-4-hydroxystyrene, poly-3-hydroxystyrene polymers,poly-3-methyl-4-hydroxy styrene, poly-2-hydroxystyrene copolymers,phenol-formaldehyde resins, polymethyl methacrylate-teriary butylmethacrylate-polymethacrylic acid terpolymers andpoly-4-hydroxystyrene-tertiary butyl methacrylate copolymers. Selectedphotoacid generators (PAG's) include, but are not limited to, MDT, fromDaychem Labs, Centerville, Ohio, onium salts, aromatic diazonium salts,and sulfonium salts.

Preferred cross-linkers include, urea-formaldehyde condensates,melamine-formaldehyde condensates and methoxylated melamine-formaldehydecondensates such as hexa-N-methoxymethylmelamine,methoxylated-dimethoyl-cresols, dimethoyl-p-cresol and1,3,5-methoxybenzene.

Preferred casting solvents include, propyleneglycol monomethyl etheracetate (PGMEA), ethyl lactate (EL), and cyclohexanone.

The preferred ranges of each of the components of a photoresistformulation in accordance with one embodiment of the present inventionare: PBS, about 1 to about 96, more preferably, about 1 to about 40,weight percent (wt %) of total solids; polymer resin, 0 to about 96,more preferably about 20 to about 96, wt % of total solids; PAG, about 1to about 20 wt % of total solids; cross-linker, about 1 to about 20 wt %of total solids; and casting solvent, 50 to 90 percent total weight ofthe formulation.

The preferred embodiment of the present invention is hereinafterdescribed in more detail by means of the following examples that areprovided by way of illustration and not by way of limitation.

EXAMPLES

1. PREPARATION OF POLYMER-BOUND ANME

24.2 grams (g) of poly-4-hydroxystyrene (PHS) is dissolved in 100milliliters of acetonitrile. 10.4 g of ANME is added to the solution.When the ANME is completely in solution, three drops of concentratedhydrochloric acid is added and the solution refluxed for seventeenhours. The reaction mixture is cooled and neutralized with aqueousammonium hydroxide. The resulting solution is added drop wise into oneliter of deionized water and stirred for thirty more minutes. Theprecipitate obtained is filtered, washed twice with deionized water anddried in vacuo.

The product was characterized by gel permeation chromatography (GPC) andultraviolet-Visible (UV-Vis) spectroscopy.

2. PROCEDURE FOR THE GPC ANALYSIS

0.25 g of PHS was dissolved in 100 g of tetrahydrofuran and to thissolution was added 0.025 g (10% by weight) of ANME. The GPC was recordedon a Waters 150C instrument using ultrastyragel columns withtetrahydrofuran as the eluting solvent. The monomeric ANME peak wasdistinct from the polymer distribution. Another sample was prepared withthe polymer-bound ANME sample prepared by the above procedure. The GPCtrace did not show the presence of any monomeric ANME but did show anincrease in the average molecular weight of the polymer.

3. PROCEDURE FOR THE UV-VIS SPECTROSCOPIC ANALYSIS

A 22 percent (%) solids solution of PHS was prepared in cyclohexanone.To this was added, ANME (10% by weight of total solids). The solutionwas cast as film on a quartz plate and the UV-Vis spectrum was recordedfrom the 200-800 nanometer (nm) range. The UV absorbance at 365 nm for amicron-thick film was 0.51. The same procedure was repeated with thepolymer-bound ANME. The UV absorbance at 365 nm was 0.49 for amicron-thick film.

4. FORMULATION OF A NEGATIVE TONE Photoresist USING POLYMER-BOUND ANME

A negative tone i-line photoresist is formulated using the polymer-boundANME (as prepared in example 1, above) as follows:

13.1 g PHS resin, 4 g polymer-bound ANME, 1.4 gbis(hydroxymethyl)p-cresol (an i-line cross-linker), 1.4 g of aphotoacid generator (e.g., trifluoromethylsulfonyl-oxobicyclo(2,2,1)-hept-5-ene 2,3 dicarboximide (MDT, available from Daychem Labsof Centerville, Ohio and 0.1 g FC-430 (a fluorocarbon surfactantavailable from 3-M Company) are dissolved in 84 g propyleneglycolmonomethylether acetate solvent (PGMEA). A 1.08 micron film is cast,baked and exposed at 365 nm. The photospeed was less than 100milliJoules per square centimeter. The exposed wafers are baked on a hotplate, followed by development in aqueous base to resolve 0.4 micronimages.

5. FORMULATION OF A POSITIVE TONE I-LINE Photoresist USING POLYMER-BOUNDANME

A positive tone i-line photoresist is formulated as follows:

13.5 g hydroxystyrene\t-butyl methacrylate copolymer, 0.5 g photoacidgenerator (e.g., MDT) and 4 g polymer-bound ANME is dissolved in 84 gPGMEA solvent. The photoresist formulation thus obtained is cast on a1.08 micron silicon wafer, soft baked, exposed, post-expose baked anddeveloped in aqueous base developer. The photoresist resolved imagescomparable to a formulation using monomeric ANME with equivalentphotospeed.

6. FORMULATION OF A NEGATIVE Photoresist USING MONOMERIC ANME

A negative photoresist formulation of 24 g ethyl-3-ethoxypropionate(EEP), 59.7 g 2-methoxy-1-propanol (Dowanol PM), 13.7 g PHS (availablefrom Maruzen America, New York, N. Y.), 1.73 g MDT, 12.1 g2,6-dimethoyl-p-cresol, 5.11 g ANME and 0.01 g FC-430 was formulated.

A 4 micrometer film was formed by spin coating at 3000 rotations perminute. A cover dish of watch glass was placed over the wafer and thewafer baked at 100° Celsius for 2 minutes. Twelve consecutive waferswere baked in this manner and the watch glass was washed with methanoland analyzed for ANME. The UV-Vis revealed the presence of depositedANME on the watch glass.

7. FORMULATION OF A NEGATIVE Photoresist USING PHS-GRAFTED ANME

In a separate experiment, the formulation of example 6, above, wasmodified to include 6.4 g of PHS-grafted ANME (containing 40 percentANME) in place of the 5.11 g of monomeric ANME. The identical processparameters were followed, and following the bake of twelve consecutivewafers, the watch glass was analyzed for anthracene groups in thedeposit and none were detected (within the sensitivity limit of 10 partsper billion).

8. RELATIVE SOLUBILITIES OF MONOMERIC ANME AND PHS-GRAFTED ANME

The solubility of monomeric ANME and of the PHS-grafted ANME was testedin 0.263N tetramethylammonium hydroxide (TMAH) and in casting solventsof propyleneglycol-monomethylether acetate (PGMEA) and ethyl lactate(EL). The following table illustrates the results that were obtained:

TABLE 1 0.263N TMAH PGMEA EL ANME insoluble insoluble soluble (2% byweight) PHS-ANME soluble soluble soluble (10% by weight) (30% by weight)(30% by weight)

A second application for a composition according to the presentinvention is as a soluble dye for deep UV positive or negativephotoresist formulations. Generally, a dye is added to a photoresistformulation to reduce the effect of image distortion caused by lateralscattered radiation from a reflective substrate. The dye is soluble indeveloper and does not leave a residue on the substrate. Thisapplication is of particular use where the lithographic process involvesvery high baking temperatures, above 140° C., where sublimation ofgreater amounts of the unbound sensitizer occurs.

A further application of the PBS of the present invention is in theformulation of an acid labile terpolymer photocomposition. The PBSfunctions as a backbone onto which acid labile groups, as described inU.S. Pat. No. 4,491,628 which is incorporated herein by reference, maybe attached. For example, when the PBS includes poly-4-hydroxystyrene(PHS) as the polymer backbone, the acid labile group is attached to thepolymer's phenolic side.

This invention has been described in terms of a specific embodiment, setforth in detail. It should be understood, however, that this embodimentis presented by way of illustration only, and that the invention is notnecessarily limited thereto. Modifications and variations within thespirit and scope of the claims that follow will be readily apparent fromthis disclosure, as those skilled in the art will appreciate.

We claim:
 1. A composition suitable for use in a photoresistformulation, said composition comprising: a monomeric sensitizerselected from the group consisting of chrysenes, pyrenes, fluoranthenes,anthrones, benzophenones, thioxanthones, and anthracenes; and abase-soluble long chain polymer having a molecular weight within therange of 1000 to 250,000, wherein the monomeric sensitizer is bound tothe base-soluble long chain polymer and wherein the base-soluble longchain polymer is selected from the group consisting ofp-hydroxystyrene-maleic anhydride copolymers,polyhydroxystyrene-p-tertiarybutyl-carbanatostyrene copolymers,poly-4-hydroxystyrene, poly-3-hydroxystyrene polymers,poly-3-methyl-4-hydroxy styrene, poly-2-hydroxystyrene copolymers,phenol-formaldehyde resins, polymethyl methacrylate-tertiary butylmethacrylate-polymethacrylic acid terpolymers, andpoly-4-hydroxystyrene-tertiary butyl methacrylate copolymers.
 2. Acomposition suitable for use in a photoresist formulation, saidcomposition comprising; a base-soluble long chain polymer having ahydroxy group; and a monomeric sensitizer bonded to the polymer, whereinthe hydroxy group of the base-soluble long chain polymer remainingintact after the monomeric sensitizer is bonded to the polymer.
 3. Thecomposition of claim 2 wherein the monomeric sensitizer comprisesbetween 1 and 25 mole percent of the composition.
 4. The composition ofclaim 2 wherein the monomeric sensitizer comprises between 1 and 15 molepercent of the composition.
 5. The composition of claim 2, wherein themonomeric sensitizer is selected from the group consisting of:chrysenes; pyrenes; fluoranthenes; anthrones; benzophenones;thioxanthones; and anthracenes.
 6. The composition of claim 5, whereinthe monomeric sensitizer includes 9-anthracene methanol.
 7. Thecomposition of claim 2, wherein the base-soluble long chain polymer isselected from the group consisting of: p-hydroxystyrene-maleic anhydridecopolymers; polyhydroxystyrene-p-tertiarybutyl-carbanatosyrenecopolymers; poly-4-hydroxystyrene; poly-3-hydroxystyrene polymers;poly-3-methyl-4-hydroxy styrene; poly-2-hydroxystyrene copolymers;phenol-formaldehyde resins; polymethyl methacrylate-teriary butylmethacrylate-polymethacrylic acid terpolymers; andpoly-4-hydroxystyrene-tertiary butyl methacrylate copolymers.
 8. Thecomposition of claim 7 wherein the base-soluble long chain polymerincludes poly-4-hydroxystyrene.
 9. The composition of claim 2 whereinthe composition is soluble in a basic developer solution.
 10. Thecomposition of claim 2 wherein the composition includes a sensitizer.11. The composition of claim 2 wherein the composition includes a dye.12. The composition of claim 2 wherein the composition further comprisesacid labile groups.
 13. The composition of claim 2, in admixure with atleast one of a polymer resin, a photoacid generator, a cross-linker anda casting solvent.
 14. The composition of claim 13 wherein: thecomposition includes an amount in the range of about 1 to about 96weight percent based on a total solids; the polymer resin includes anamount in the range of about 0 to about 96 weight percent based on thetotal solids; the photoacid generator includes an amount in the range ofabout 1 to about 20 weight percent based on the total solids; and thecross-linker includes an amount in the range of about 1 to about 20weight percent based on the total solids.
 15. The composition of claim14, wherein: the composition includes an amount in the range of about 1to about 40 weight percent based on a total solids; and the polymerresin includes an amount in the range of about 10 to about 96 weightpercent based on the total solids.
 16. The composition of claim 2,wherein a molecular weight includes from about 1,000 to about 250,000.